Making Soap - Saponification

 

Objectives

 

The objective of this laboratory is to make lye soap via the saponification reaction.

 

 

Background

 

Soap making has remained unchanged over the centuries. The ancient Roman tradition was to take rain water, potash and animal tallow, turning it into a cleansing agent. There are many legends about how soap was discovered.  Some say that after a heavy rain fall on the slopes of Mount Sapo, the water mixed with the animal fat and ashes around an important sacrificial alter.  According to legend this trickled down to the banks of the Tiber River where washer women noticed that the substance made their job easier and the wash cleaner.

 

Making soap was a long and arduous process.  First the fat had to be rendered, that is melted and filtered to remove any non-fat solids.  Then the potash solution was added to the hot fat.  Since water and oil do not mix, this mixture had to be continuously stirred and heated sufficiently to keep the fat melted.Slowly a chemical reaction called saponification would take place between the fat and the hydroxide which resulted in a liquid soap.  When the fat and water no longer separated, the mixture was allowed to cool.  At this point salt, such as sodium chloride, was added to separate the soap from the excess water.  The soap came to the top, was skimmed off, and placed in wooden molds to cure.  It was often aged many months to allow the reaction between the fat and hydroxide to run to completion.  Poorly make soap could contain excess alkali and could dry and chap people's skin.  Today laundry soaps such as "Fels Napha" soap are much like the home made soaps made by early Americans.

 

All soap is made from fats and alkaline solutions. There are many kinds of fats, both animal and vegetable.  Animal fats are usually solid at room temperature, but many vegetable fats are liquid at room temperature. Liquid cooking oils are made from vegetable fats extracted from corn, peanuts, olives, soybeans, and many other plants. When it comes to making soap, however, all different types of fats (anything from lard to exotic tropical plant oils) can be used!

 

Basic (alkaline) solutions all contain a metal and a hydroxide ion. The most common bases are those produced by the reaction of a group I metal plus water.  These are highly water-soluble and can be used to make very strong solutions.  Lye and drain cleaner are the most common alkali compounds used in every day life.

 

Up until the early 1900's, many people made their own soap from household waste products.  They used the solid animal fats that were left over from cooking and a potash solution from wood ashes. 

 

Today homemade soaps are able to duplicate or improve on most commercial products. The use of scents, coloring agents, herbs and more are within the scope of home soap makers who are also allowed to choose exactly what will go into their product. Many home soap makers today make their own dish liquid, laundry soaps, bath soaps, facial soap and liquid body soaps. Even though all soap is made by the chemical combination of lye, water and fat; which is called saponification, soaps differ from one another depending on the type of fat and lye that is used and the amount. For instance, lye made from wood ash produces a soft soap. Commercial lye produces a hard soap. Soap with coconut oil will lather well in cold water but tend to dry the skin. Soaps that contain excess amounts of unsaponified fat are very gentle and make good facial or toilet soaps. 

 

Today soap is made in basically the same way, but we can use a few tricks of chemistry to accelerate the process.  We will start with a liquid vegetable fat [cooking or salad oil] and use alcohol to speed the process of mixing oil and a water-based solution of sodium hydroxide.

 

Saponification Reactions:

 

Fat  +  Lye  →  Soap  +  Glycerol

saponification reaction

 

 

Procedure

 

Safety

 

First, be sure to exercise caution when heating the soap solution.  The alcohol vapors can ignite!  Thus, use gentle heating ONLY throughout this lab.    If either of these chemicals comes into contact with your skin, immediately rinse with water for a minimum of fifteen minutes and notify your instructor. 

 

Materials and Equipment

 

Materials: fat [cooking oil, linseed oil, shortening], ethyl alcohol, 6M sodium hydroxide solution [lye], saturated sodium chloride solution

 

Equipment: 250 mL beaker, ring stand, wire mesh, Bunsen burner, stirring rod

 

Part A: Generating and Collecting Oxygen Gas

 

1.     Place 10 ml of cooking oil, linseed oil, or melted vegetable shortening into a 250 mL beaker.

 

2.    Add 10 ml of ethyl alcohol (ethanol) to the beaker.

 

3.    Add 5 ml of 6M sodium hydroxide solution to the beaker.

 

4.   Heat GENTLY with constant stirring.  YOU MUST STIR DURING THE ENTIRE HEATING PROCESS UNTIL THE SOAP IS THICK ENOUGH TO POUR INTO THE MOLD. Add dye to mixture, if color is desired.  NOTE:  If flame is too high, the alcohol vapors will ignite, followed by the cooking oil.  

 

5.   Continue heating until all oil droplets have been dissolved.  The mixture froths (foam) easily.  (Obtain a thick lotion consistency.)  If this happens, stir the foam to break up the bubbles so that it does not overflow.  The heating process usually takes about 10 minutes.

 

6.    Allow the mixture to cool.  Add desired fragrance.  The substance will be semi-solid. 

 

7.    After the mixture has cooled, remove the impurities (sodium hydroxide and glycerol) by adding 20 ml of hot water to the beaker.  Stir to dissolve the substance (the crude soap).

 

8.   Finally, re-precipitate the soap with salt by adding 25 ml of sodium chloride solution to the mixture.  Do not stir.  If it does not seem to mix, swirl it gently once or twice.

 

9.   Place resulting soap in a chosen mold and allow to dry.

 

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